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The discovery that nitric oxide (NO) has powerful vasoactive properties identical to those of endothelial-derived relaxing factor spawned a vast body of research investigating the physiological actions of small gas molecules. NO, which arises endogenously through the action of nitric oxide synthase (NOS) enzymes, is a highly reactive gas that plays important roles in the regulation of vascular and immune function. Carbon monoxide (CO), a similar yet much more chemically stable gas, occurs in nature as a product of the oxidation or combustion of organic materials. CO also arises in cells and tissues as a byproduct of heme oxygenase (HO) activity, which degrades heme to biliverdin-IXalpha. Like NO, CO acts as a vasorelaxant and may regulate other vascular functions such as platelet aggregation and smooth muscle proliferation. CO has also been implicated as a neurotransmitter in the central nervous system. HO-1, the inducible form of HO, confers cytoprotection against oxidative stress in vitro and in vivo. CO, when applied at low concentration, exerts potent cytoprotective effects mimicking those of HO-1 induction, including down-regulation of inflammation and suppression of apoptosis. Many of the effects of CO depend on the activation of guanylate cyclase, which generates guanosine 3',5'-monophosphate (cGMP), and the modulation of mitogen-activated protein kinase (MAPK) signaling pathways. This review highlights new advances in the interaction of CO with cellular signaling processes.